1. Field of Invention
This invention relates to an optical disk test system for measuring the mechanical characteristics of optical disks.
2. Description of Prior Art
The mechanical characteristics of an optical disk concern the shape thereof, such as axial runout or concentricity, and are associated with how effective focusing and tracking servos are. As to the focus servo, the thickness of the substrate is important in relation to aberration, and the axial runout and its high frequency component, ie. acceleration, is important from the view point of controllability. As to the tracking servo, the tilt, concentricity and radial acceleration (i.e. circularity) are important factors.
FIG. 1 depicts a conventional optical disk test system comprising an optical disk 1; a spindle motor 2 for clamping and rotating disk 1 at a constant speed; and a measuring head 3 comprising focus servo and tracking servo mechanisms for causing the focal point of a laser beam irradiating disk 1 to follow the guide groove of disk 1, and a displacement detector for optically detecting the displacement of a focusing lens 31 driven by the focus and tracking servo mechanisms. Measuring head 3 generates an output signal proportional to the displacement of focusing lens 31. Numeral 4 denotes measuring head 3 being in the radial direction of disk 1.
The test system functions similar to a reproducing system, and focusing lens 31 is subjected to servo control in a focusing direction F and in a tracking direction T so that the position in the reproducing state of the guide groove of disk 1 may be irradiated with the laser beam. Thus, if the displacements in the two directions F and T of focusing lens 31 at this time are detected, it is possible to measure the magnitudes of the axial runout and concentricity of disk 1. Noting the displacement in the focusing direction, for example, it is possible to measure the axial runout, the runout acceleration, the tilt and the substrate thickness. Noting the displacement in the tracking direction, it is possible to measure the concentricity and radial acceleration (i.e. the roundness).
For detecting the foregoing displacements of focusing lens 31, the drive current of a lens actuator for displacing focusing lens 31 is utilized, or a differential transformer or a capacity type displacement sensor is used. In case the displacement of focusing lens 31 is detected by making use of the drive current as above, the detection is affected by the frequency characteristics, hysteresis and frictional force of the lens actuator so that the displacement of focusing lens 31 cannot be accurately detected.
Since the differential transformer or capacity type displacement sensor is a detector detecting a one dimensional displacement, two independent detectors must be used for detecting the displacement of focusing lens 31 in the focusing direction F and in the tracking direction T. Since interference occurs between the servo mechanisms of the focusing and tracking directions F and T, the outputs of the individual detectors must be corrected so as to eliminate these influences.
It can thus be appreciated that the conventional test systems have many deficiencies and disadvantages.